Q: gap between visits of more than 3 months- sql i am trying to identify the id's which are having gap between visits of more than 3 months

CREATE TABLE Data (
    ID int,
    visit datetime);

INSERT INTO data (ID,visit)
VALUES
    (1,'1998-05-18 00:00:00.000'),
    (1,'1995-05-18 00:20:00.000'),
    (1,'1994-12-18 01:00:00.000'),
    (1,'1994-007-18 00:00:00.000'),
    (2,'2000-06-29 13:30:00.000'),
    (2,'2000-05-29 13:45:00.000'),
    (2,'2000-04-29 10:30:00.000'),
    (3,'2009-09-29 13:30:00.000'),
    (3,'2009-01-29 13:30:00.000'),
    (3,'2008-12-29 13:30:00.000');


A: You can use lag().  date/time functions differ significantly among databases, but the idea is:
select distinct id
from (select d.*, lag(visit) over (partition by id order by visit) as prev_visit
      from data d
     ) d
where prev_visit < visit - interval '3 month';

Here is a db<>fiddle.  This uses SQL Server, so the where clause is:
where prev_visit < dateadd(month, -3, visit);

